Her mother, Trijn, a communist bringing up her children independently in the Dutch city of Haarlem, sheltered Jews, dissidents and gay people as they fled Germany in the 1930s. Oversteegen, who was seven when Adolf Hitler came to power, bunked in with her big sister Truus to make room.

It was the start of a struggle that would last until she died on 5 September, the day before her 93rd birthday, in a nursing home not far from where, as teenagers, she and Truus carried out a campaign of assassinations and sabotage against Nazi invaders with pistols hidden in their bicycle baskets.

“If you ask me, the war only ended two weeks ago,” her son Remi Dekker told the Observer. “In her mind it was still going on, and on, and on. It didn’t stop, even until the last day.”

Oversteegen’s war began one Friday in May 1940 with planes roaring overhead and the smell of smoke. Realising the Nazis had invaded the Netherlands, her family began burning their radical literature. Oversteegen, then 14, and Truus, 16, were already used to smuggling refugees and distributing forbidden texts. It wasn’t long before the resistance came to recruit them. Her mother only gave them one rule, Oversteegen once recalled: “Always stay human.”

Oversteegen in 1943, when her innocent appearance made her an invaluable resistance fighter.

Oversteegen was petite, and with her twin plaits she easily passed for 12. Her innocent looks made her invaluable as she could slip by Nazi controls unnoticed. The two sisters began as couriers, moving weapons and stealing identity papers to help Jewish people escape.

One early assignment was arson – the two burned down a Nazi warehouse, flirting with the guards as a distraction. Soon, they were taken to an underground potato shed and taught how to shoot. Their method was the Dutch equivalent of a drive-by. “My mother drove the bicycle, and Freddie sat on the back and was shooting,” recalled Truus’s daughter, Hannie Menger. “Because they were girls, nobody noticed them.”

Some targets were Dutch collaborators who gave the Nazis details of Jewish and dissident families. Others were high-ranking Nazi officers. On one assignment to “liquidate” a member of the SS, the sisters found their target in a restaurant. As Oversteegen kept watch, Truus lured him out by seductively proposing a walk in the forest, where another resistance fighter shot him dead. The sisters regarded the killings as a grim necessity that was secondary to their more important work: rescuing children. The two helped smuggle Jewish children through the Netherlands, sometimes even as bombs fell from Allied aircraft overhead.

Occasionally, they were not successful and children were killed. This caused grief so profound that the sisters’ descendants struggle to speak about those operations today.

Their cell expanded to include Hannie Schaft, a law student notorious to Nazi authorities as “the girl with the red hair”. Truus, a commanding presence with a level gaze and a throaty laugh, was the leader. The three developed an iron bond.

Oversteegen never forgot the day that Schaft failed to return from an assignment. She had been captured at a checkpoint. Though her hair was dyed black, the red colour of her roots revealed her identity. Aged 24, Schaft was executed in the dunes west of Haarlem, just 18 days before the Netherlands was liberated.

“Hannie was her soulmate friend,” said Manon Hoornstra, a filmmaker to whom Oversteegen confided many of her war memories. “Freddie could never understand why the Nazis killed her just before the end of the war. She always took red roses to her grave.” The peace was not easy for Oversteegen. As the cold war began and McCarthyism took hold in the United States, communist resistance fighters became so out of favour that in 1951 the Dutch government forcibly tried to prevent the commemoration of Hannie Schaft’s death. Oversteegen felt alienated from the country she had fought for, and was embittered to see former Nazi sympathisers not held to account.

German soldiers marching through a town in Holland in May 1940. Photograph: Hulton Archive/Getty Images

She married Jan Dekker, an engineer at a local steel factory, and threw herself into domestic life, raising three children. Her family tried to protect her from troubling memories of the war, but she struggled with the trauma all her life, particularly around the annual Remembrance of the Dead on 4 May.

“She shot a few people, and these were the real, real bad guys,” her son, Remi Dekker, recalled. “But she hated it, and she hated herself for doing it.”

Unlike Truus, who married a fellow resistance fighter and was open about her experiences, Oversteegen struggled to speak about the war and sometimes felt overlooked.

She feared the very attributes that served her so well in the war – her tiny stature and sweet voice – made her invisible in peacetime. “She hated her high voice,” Dekker said. “She used to say, ‘Nobody listens to me!’” The release of a film about Hannie Schaft in 1981 made her famous and helped rehabilitate the sisters in national memory. Eventually in 2014, the Dutch government awarded them medals of military service, in what Mark Rutte, the prime minister, called “an act of historical justice”.

The two sisters retained a deep bond until Truus’s death two years ago. “One word was enough for them to understand each other,” recalled Menger, Truus’s daughter. “They had relied on each other completely during the war. Their lives were in each other’s hands.”

Towards the end of her life, Freddie began to speak about her war experiences, opening up to the documentary makers Hoornstra and Thijs Zeeman for their 2016 film Two Sisters in the Resistance. After much persuading, she returned to the woods where Truus had led the SS officer to his death and where, Freddie believed, he is still buried.

“On the way there in the car, we could see that she was very vulnerable because she started singing,” Hoornstra recalled. “It was a song that the members of the resistance group always sang when they were afraid.”